1. Field of the Invention
This invention relates to a color video camera, and in particular to a camera of the type employing a single color image sensor providing interlaced scanning of luminance and chrominance information for both fields of a television frame.
2. Background of the Invention
Luminance and chrominance information can be obtained from a single solid-state sensor by covering the photosites forming the sensor with a repetitive array of color filters. Since luminance is a combination of color information with a strong bias toward green, it is frequently based on just the green information provided by the sensor. The chrominance (color) content is frequently based on differences between the luminance (i.e., green information) and other primary colors (i.e., red and blue information). Each separate color is isolated by passing the scene light through an appropriate color filter before it strikes the sensor. As disclosed in "Color Filters and Processing Alternatives for One-Chip Cameras," by K. A. Parulski (IEEE Trans. on Electron Devices. ED-32, No. 8, August 1985, pp. 1381-1389), many different color filter array patterns have been used in single-sensor color video cameras. One example is the "Bayer" checkerboard pattern, which is disclosed in U.S. Pat. No. 3,971,065. The basic Bayer pattern is shown in FIG. 1; when interlaced scanning of two fields is desired, the modified Bayer pattern as shown in FIG. 2 is used.
Due to the interlaced scanning employed in present video standards such as NTSC and PAL, the color filter array patterns on video sensors are normally designed so that the image sampling is the same for each of the two video fields. For example, the odd field sampling is identical to the even field sampling for the prior art pattern of FIG. 2. The use of interlaced readout, however, does not require that this sampling be the same, but only that each field include both red and blue photoelements. An interlaced image sampling pattern can still be obtained if the chrominance sample positions are shifted from one field to another. For instance, U.S. Pat. No. 4,479,143 discloses a further modified Bayer pattern (see FIG. 4) in which red and blue sensor elements are provided at the same sampling rate as the green sensor elements, when horizontal alignment is considered. This arrangement provides improved chrominance resolution in the horizontal direction.
The color filter shown in FIG. 3 is representative of a newer type of filter used in a single sensor camera for improved image quality. In this filter, which is described in relation to a camera disclosed in U.S. Pat. No. 4,663,661, the green information (luminance) is sampled three times as frequently as red and blue information because it contributes more significantly to image resolution. Accordingly, an image sensor using this filter contains three green "luminance" pixels for every one red or blue "chroma" pixel. One problem is the occurrence of some amount of color edge artifacts (also referred to as aliasing) on horizontal chrominance edge transitions. Because the red and blue photosites only occur in every 4th column of the sensor, a linear interpolation used to form the interpolated red and blue signal values from the nearby red, green, and blue sensor values can create some slight artifacts on colored edges.